A thin-film transistor (TFT) is one type of field effect transistor (FET), and it is basically constituted of three terminals of a gate, a drain and a source and is mainly used for a liquid crystal display. Further, the thin-film transistor is also used for a semiconductor device mounted together with a normal MIS (metal insulator semiconductor) transistor, for example, an SRAM (static random access memory) and a DRAM (dynamic RAM).
Japanese Patent Application Laid-Open Publication No. 5-190856 (Patent Document 1) and Japanese Patent Application Laid-Open Publication No. 6-21458 (Patent Document 2) describe that, when a thin-film transistor is used for a memory cell of an SRAM, the off current of the thin-film transistor affects the standby current of the SRAM, and it is effective for the reduction of the standby current to reduce the thickness of a channel of the thin-film transistor. Further, the Patent Documents 1 and 2 describe a thin-film transistor constituted of the three terminals of a source, a drain and a gate formed on an insulator layer and a channel formed on the gate via a gate insulator. The thickness of the channel is thinner than that of the source and the drain.